The main purpose of an antenna is to control a wave front at the boundary between a source (e.g., a feed probe) and the medium of propagation (e.g., air). An antenna enables the radiation of electromagnetic (EM) energy from the source into the medium of propagation. The radiation of EM energy has been accomplished in a number of ways through the use of antennas of various sizes and configurations.
A common waveguide antenna is the slot or aperture antenna. The slot antenna is typically constructed from a conductive material having one or more slots. The slot antenna radiates EM energy into the propagation medium from each slot in the conductive material. When current is introduced to the conductive material, the slot disrupts the current flow causing an electric field to be induced across the area including the slot.
Slot antennas can be implemented as a slot cut into the conductive surface of a parallel planar plate waveguide comprising two parallel conducting planar plates separated by a dielectric slab of uniform thickness. Parallel planar plate waveguides provide a means of propagating EM energy and directing the energy to a radiator. Where a slot is cut into the parallel planar plate waveguide, the slot is the radiator. The size of the slot determines how much EM energy will be radiated.
In many antenna applications (e.g., telecommunications and radar), it is necessary to design antennas with good directive characteristics to meet the demands of the long distance communications required by the particular application. This can be accomplished by increasing the electrical size of the antenna. One means of increasing an antenna's electrical size is to enlarge the dimensions of the antenna's radiating components. Another common means is to form an assembly of radiating elements in an array. The individual radiating elements of an array may be of any form (e.g., wires or slots) and the resulting radiation pattern of the array is an aggregate of the individual elements' radiation patterns.
When rapid beam scanning or multiple beams are required, phased arrays are often used. Although planar arrays are common, multiple array faces are required to generate radiation patterns of 360.degree. in the azimuth plane. Cylindrical arrays can be used to generate such radiation patterns. However, in practical applications, the radiation patterns of the individual elements of the cylindrical array interfere such that the radiation pattern of the array may be less than ideal. Moreover, the cylindrical array typically uses a complex lossy feed network to commutate the excitation around the cylinder and only some of the elements are used at a given scan angle making power handling more difficult and increases the sensitivity to error. At frequencies above 30 GHz, the design of planar and cylindrical arrays of discrete radiators becomes more difficult in that while the available area per element becomes quite small, each element must be equipped with a variety of support components, such as radiating elements, phase shifters, attenuators, dc power distribution, connectors, logic circuits, etc.
One variation on the conventional parallel planar plate waveguide is the geodesic parallel plate waveguide. A geodesic parallel plate waveguide can be created by forming a parallel plate waveguide from conformal structures, such as a pair of cylinders, made from a conductive material. More specifically, by placing a cylinder of conductive material within another cylinder of conductive material, a parallel plate waveguide can be formed with each cylinder representing the opposing plates of the waveguide. The parallel plate waveguide formed thereby has no side walls. Because the geodesic waveguide is circumferential, it can scan a 360.degree. radiation pattern in the azimuth plane. Furthermore, it is superior to the cylindrical array, in that it can be fed from a smaller feed region. Unlike the cylindrical array, the EM energy from the input feed of the geodesic cylinder is simultaneously phased and spatially distributed to form the radiation pattern. The additional components required by the cylindrical array are thus eliminated or minimized.
The essence of the geodesic structure is that the EM energy is forced to follow geodesic paths between the parallel plates. EM energy will follow the most direct path between two points. The use of the geodesic parallel plate structure and phased feed probes provides a well focused radiation pattern in azimuth. These benefits are a result of the propagation of EM energy through the structure
However, while previously manufactured geodesic antennas have provided good radiation pattern characteristics in the azimuth plane, they have failed to provide the ability to generate a shaped pattern in the elevation plane. Current geodesic antennas have failed to provide a shaped pattern in the elevation plane, because they have been designed to produce a radiation pattern at a single annular opening at the top-most portion of the conformal structure (i.e., where the parallel plates terminate). Attempts at controlling the elevation pattern of these geodesic antennas include locating horns, reflectors, lenses, and line sources at the single output opening. While these control means are effective for focusing a beam in the elevation plane, they are ineffective for shaping a radiation pattern in the elevation plane. These modifications extend the vertical height of the antenna and greatly increase the horizontal dimension if a small flare angle is used for the horn aperture. In applications, such as telecommunications, the desired radiation pattern of a geodesic antenna may differ depending on the demands of a particular market. The control means listed above are incapable of providing the control ability necessary to accommodate the various desired radiation patterns.
Moreover, current geodesic antennas also tend to produce spurious rays of EM energy, because the physical structure of the geodesic antenna supports a multitude of ray paths between a feed point and the radiation element. Spurious rays can produce destructive interference with the desired ray paths. This causes undesirable ripples in the pattern associated with a given feed port which degrades the azimuth pattern when all feed probes are simultaneously excited.
Therefore, there is a need for a geodesic antenna that is capable of forming a focused narrow beam, omni pattern beam, or sector shaped beam in the azimuth direction. The antenna should also be capable of generating a radiation pattern with shaped coverage in the elevation plane and should provide a high degree of control over the shape of the elevation plane radiation pattern. The antenna should minimize the generation of spurious rays of EM energy. Furthermore, there is a need for a geodesic antenna that is designed such that it is inexpensive to manufacture and minimizes the need for additional components, while being adaptable to changing radiation pattern requirements.